PHP Operators: Arithmetic, Logical, String and More

An operator takes one or more values and produces a new one: $a + $b adds, $a . $b joins strings, $a && $b is a logical AND, $a === $b compares, $i++ increments.

<?php
$a = 17;
$b = 5;

echo "a + b  = ", $a + $b, "\n";
echo "a - b  = ", $a - $b, "\n";
echo "a * b  = ", $a * $b, "\n";
echo "a / b  = ", $a / $b, "\n";
echo "a % b  = ", $a % $b, "\n";
echo "a ** 2 = ", $a ** 2, "\n";
echo "intdiv = ", intdiv($a, $b), "\n";

/ returns a float unless two integers divide evenly: 17 / 5 is 3.4, 10 / 5 is the integer 2. When you want the whole-number part only, use intdiv(). Rounding is covered on the [math page](/docs/php/math).

Arithmetic operators

Modulo is useful for "every Nth item" and even/odd checks. Its result takes the sign of the left side, and dividing by zero throws an error you can catch:

<?php
for ($i = 1; $i <= 6; $i++) {
    echo $i, $i % 2 === 0 ? " even" : " odd", "\n";
}

var_dump(-7 % 3);
var_dump(fmod(7.5, 2));   // modulo for floats

try {
    echo 10 % 0;
} catch (DivisionByZeroError $e) {
    echo "Error: ", $e->getMessage(), "\n";
}

Assignment operators

= stores a value. Every arithmetic and string operator has a combined form that updates a variable in place.

<?php
$total = 100;
$total += 20;   // $total = $total + 20
$total -= 5;
$total *= 2;
$total /= 10;
$total **= 2;
echo $total, "\n";

$path = "/home";
$path .= "/docs";      // append a string
echo $path, "\n";

$name = null;
$name ??= "Guest";     // assign only if null or unset
echo $name, "\n";

??= belongs to the null coalescing operator family. It assigns only when the variable is unset or null.

Increment and decrement: ++ and --

$i++ returns the current value, then adds 1. ++$i adds 1, then returns the new value. The difference only shows when you use the result in the same expression.

<?php
$i = 5;
echo $i++, "\n";  // prints 5, then $i becomes 6
echo $i, "\n";

$j = 5;
echo ++$j, "\n";  // $j becomes 6, prints 6

$k = 10;
$k--;
echo $k, "\n";

// ++ also works on letters
$col = "A";
$col++;
echo $col, "\n";
$col = "Z";
$col++;
echo $col, "\n";

The letter trick is how spreadsheet column names (A, B, ... Z, AA) are often generated.

Logical operators: &&, ||, ! and xor

&& and || short-circuit: the right side is not evaluated when the left side already decides the result. That is what makes isset($user) && $user->isAdmin() safe.

<?php
$age = 20;
$hasTicket = true;

var_dump($age >= 18 && $hasTicket);
var_dump($age < 12 || $age > 65);
var_dump(!$hasTicket);
var_dump(true xor true);

function check(string $label): bool {
    echo "checked $label\n";
    return true;
}
false && check("A");   // never runs
true || check("B");    // never runs
true && check("C");    // runs

and vs &&: the precedence trap

and and or bind more loosely than =, so in an assignment they apply after the value is stored. This is a classic bug:

<?php
$x = true and false;   // read as: ($x = true) and false
var_dump($x);

$y = true && false;    // read as: $y = (true && false)
var_dump($y);

$z = (true and false); // parentheses fix it
var_dump($z);

The low precedence is useful in one idiom, $file = fopen($path, 'r') or die('Cannot open');, where you want the assignment to happen first. Everywhere else, use && and ||.

String operators: . and .=

PHP has two string operators: . joins two strings ("Grace" . " " . "Hopper") and .= appends to a variable ($html .= "<li>"). + never joins strings: "3" + "4" is the number 7, while "3" . "4" is the string "34". The strings page covers interpolation and quotes.

Comparison and the spaceship operator <=>

== compares after converting types, === also checks the type, and <=> returns -1, 0 or 1, which is exactly what sort callbacks need. The details and the full loose comparison table are on the comparison operators page.

<?php
var_dump(1 <=> 2);
var_dump(2 <=> 2);
var_dump(3 <=> 2);

$prices = [30, 5, 120, 18];
usort($prices, fn($a, $b) => $a <=> $b);
echo implode(", ", $prices), "\n";

usort($prices, fn($a, $b) => $b <=> $a);   // descending
echo implode(", ", $prices), "\n";

Array operators

+ on arrays is a union: keys from the left array win and only missing keys are added from the right. That surprises people who expect it to append. == checks the same key/value pairs, === also requires the same order and types.

<?php
print_r([1, 2] + [3, 4, 5]);
print_r(["a" => 1] + ["a" => 99, "b" => 2]);

var_dump(["a" => 1, "b" => 2] == ["b" => 2, "a" => 1]);
var_dump(["a" => 1, "b" => 2] === ["b" => 2, "a" => 1]);

To append one list to another, use array_merge() or the spread syntax [...$a, ...$b].

Operator precedence

When an expression mixes operators, PHP applies them in a fixed order. The ones that trip people up, with what PHP 8.3 prints:

echo 2 + 3 * 4;        // 14: * before +
echo (2 + 3) * 4;      // 20
echo 2 ** 3 ** 2;      // 512: right to left, 2 ** 9
echo -2 ** 2;          // -4: ** before unary minus
echo "Sum: " . 2 + 3;  // Sum: 5, since PHP 8 + runs before .

When in doubt, add parentheses. They cost nothing and the next person reading the line does not have to remember the table.